Water purification by the use of ion exchange resins has been practiced since the late 1940s. Cationic mineral contaminants such as Na+1, Ca+2, and Mg+2 are removed by a cation exchange resin. Anionic mineral contaminants such as Cl−1, SO4−2 are removed by an anion exchange resin. Non-ionic species, such as CO2 and silica, are also removed by an anion exchange resin; these species become anionic upon passage into the high pH environment of the anion resin.
Ion exchange, for the most part, is a batch process. The resins are in separate vessels for a primary demineralizer and the charged contaminants are removed during a service cycle. When the resins become saturated and no longer capable of purifying water, they are taken off-line and subjected to a rejuvenation process termed regeneration. Cation resins are regenerated with ambient dilute acid solutions; anion resins are treated with warmed dilute caustic solutions. These regenerant solutions strip off the service ions, allowing the resins to then be used over again in another service cycle.
Many demineralizer systems are installed in water plants using surface supplies as their source of raw water. This includes rivers, streams, and lakes. (Ion exchange cannot be used on sea water.) Surface waters are prone to contamination with “natural organics,” a broad class of compounds arising from the microbial degradation of leaves and pine needles dropping into the surface sources. The organics typically impart a yellow or yellow-brown caste to the water.
The chemistry of natural organics is complicated and a great deal of research has been dedicated to the elucidation of their structure, mostly associated with the use of surface waters for potable applications. For industrial purposes, it is sufficient to describe the organics as follows:                Broad range of molecular weights (up to several million Daltons);        Overall negative charge, due to the presence of COOH (carboxylic acid) groups;        Complicated structure containing aromatic and aliphatic sub-structures;        Presence of imbedded Fe ions within the structures, most likely by a chelation-type mechanism; and        Geographical and seasonal variations in structural details.        
Organic fouling occurs as the anion exchange resins remove the organics from the inlet water, but fail to release the organics during regeneration. Although the per-cycle loading is in the ppm (parts per million) range, the operation of the system over many dozens or hundreds of service/regeneration cycles results in the accumulation of a high level of fouling on the resins.
Organic fouling has a direct impact on the efficient operation of a demineralizer. There are two main mechanisms: (1) blockage of active groups, and (2) prolongation of the regeneration final rinse. The organics are large molecules, with multiple points of attachment to the ion exchange resin's active sites. They can block access of the normal service ions (Cl−1, SO4−2, etc.), leading to a shortened run. In addition, the organics trapped within the polymeric structure of the resin absorb Na+1 ions from the regenerant caustic, forming COONa. The latter slowly hydrolyzes, releasing Na+1 ions into the final rinse step of the regeneration process. The end of the final rinse is predicated on a drop in conductivity to a pre-determined value, typically <15 μS. A high Na+1 background will raise the conductivity, prolonging the rinse step.
During the final rinse, service water goes through the cation and anion vessels. The final rinse is “service to sewer,” because the water is discarded. A prolongation of the final rinse, however, subtracts time from the next service cycle. Quite often, a resin vessel which requires an overly long final rinse will give a shortened service cycle directly after. A shortened service cycle requires more frequent regenerations to purify a given amount of water, thus increasing the consumption of acid and caustic used in the regeneration process.
Ion exchange is also used in non-water treatment related applications, such as the decolorization of liquid sugar solutions and the removal of unwanted acidic and basic species from organic product streams. Decolorization resins become fouled with materials similar to naturally occurring organic foulants, but, due to their higher concentration in the sugar solution, fouling occurs more rapidly and to a greater extent than in conventional water treatment. The resins used in these applications are very expensive and, prior to this patent application, it is believed that no effective cleaning protocol had been developed.
There have been many attempts to remove the organic fouling from anion resins. Most revolve around the use of strong brine solutions, typically 10% or more. In the presence of these solutions, the organics are induced to leave the resin and diffuse out into the brine. This can be readily seen even within a few minutes of contacting of resin and brine: the solution quickly becomes tinged with a yellow or orange or red-brown. The variations in color are believed to arise from geographical variations in the exact structure of the organics.
The evolution of color has been used as a measure of the level of fouling. After a 24-hour exposure, usually to a warmed brine solution, the color in the brine can be compared against a VCS (Varnish Color Standard) chart, which documents colors from a VCS of #1, which is water white, to a VCS #18, which is opaque black. Heavy organic fouling usually generates a VCS of #8 to #16.
An alternate method is to measure the TOC (Total Organic Carbon) in the brine solution. This requires a very sophisticated analytical instrument, of which there are several brands on the market. Heavy organic fouling is usually indicated by a TOC value in the brine of 1500 to 6000 ppm.
Unfortunately, all the methods based on the above basic approaches are “method dependent,” in which the test value depends on the method used. This makes inter-laboratory comparisons difficult, although each lab's results can be internally consistent.
The evolution of the color from the resin into a brine solution also provides the basis for a cleaning procedure. Practitioners throughout the resin industry have published many procedures based on the use of brine by itself or in combination with caustic (NaOH). Some of the procedures are quite elaborate, with multiple soaking periods in between brining steps.
Results of the use of brine/caustic are highly variable, ranging from a minimum of 10% removal to 90% removal, with even optimized procedures providing only variable results, typically 50-80% removal.
Accordingly, a need exists for an improved cleaning method for ion exchange resins.